1. Field of the Invention
The present invention generally relates to power modules and particularly relates to a power module in which at least one power device chip and circuits for driving and controlling the power device chip are incorporated within one and the same package.
2. Description of the Related Art
A power module, which is small in size and excellent in workability, and in which at least one power device chip, for example, a power transistor chip, an MOS device chip, a diode chip, a thyristor chip, or the like, and circuits for driving and controlling the power device chip are incorporated within one and the same package has been used widely as an invertor power source circuit for composing a three-phase alternating current wave, or a power source circuit for controlling peripheral equipments.
Since the power device chip generates heat, therefore, there is a case where the heat causes the circuit elements of the built-in circuits for driving and controlling the power device chip to make erroneous operation or to get in failure. In order to protect the circuits for driving and controlling the power device chip from the heat of the power device chip, therefore, the circuits for driving and controlling the power device chip must be separated at a certain extent from the power device chip, and there is a limit in miniaturization of the power module.
There has been therefore proposed a power module having a two-layer inside structure to reduce the area occupied by the power module, as shown in FIGS. 4 and 5. That is, a power module 11 has a two-layer structure of a base substrate 13 and a control substrate 14. The base substrate 13 is constituted by an aluminum substrate or a DBC (direct bonding copper) substrate for supporting power device chips, for example, power transistor chips 12. On the other hand, the control substrate 14 is disposed at a distance above the base substrate 13 and constituted by a printed circuit substrate having printed patterns of circuits for driving and controlling the power transistor chips. An output terminal 15 of the power module is connected to the upper surface of the base substrate 13, and patterns are provided on the base substrate 13 for connection with input/output terminals of the power device chips 12 and the control circuit substrate 14. The control circuit substrate 14 is supported, at a distance above the base substrate 13, by connection terminals 16 which act also as support legs. The input/output terminals of the control circuit substrate 14 are connected to the base substrate 13 through the connection terminals 16. Circuit elements 17 of the circuits for driving and controlling the power transistor chips 12 are mounted on the control circuit substrate 14, and a control terminal 18 is also connected to the control circuit substrate 14. The base substrate 13 and the control circuit substrate 14 are covered with a case cover 19 of synthetic resin. The base substrate 13 is exposed at its lower surface. Alternatively, a heat sink plate may be attached onto the lower surface of the base substrate 13 if necessary.
Each transistor chip 12 employs a bare chip in which a base electrode 20 and an emitter electrode 21 are formed on its upper surface, while its lower surface is formed as a collector terminal 22 as a whole, as shown in FIG. 5. The lower surface, that is, the collector terminal 22, of this transistor chip 12 is fixed, through solder 24, onto the upper surface of a heat spreader 23 of metal such as copper having a high heat conductivity, and the lower surface of the heat spreader 23 is fixed onto the base substrate 13 through solder 25. The base and emitter electrodes 20 and 21 of the transistor chip 12 are connected to predetermined patterns 26 and 27 on the upper surface of the base substrate 13 by the wire bonding with aluminum wire 28. The collector terminal 22 is connected to a predetermined pattern (not shown) on the base substrate 13 through the solder 24 and wire bonding (not shown). The transistor chip 12 fixed on the base substrate 13 is coated with a sealing agent 29 of synthetic resin, as shown in FIG. 4.
Having such a two-layer structure, the power module can be reduced in its area, but it increases its height. Further, there is a defect that it is necessary to provide means for supporting the control circuit substrate 14 above the base substrate 13 and means for electrically connecting the base and control circuit substrates to each other. The structure is therefore made complicated, and the number of assembling steps increases. Further, the base substrate 13 is constituted by an aluminum substrate or a DBC substrate from the point of view of requiring high heat dissipation and insulation, resulting in increased material cost.